I'm trying to get up to speed with using WPF and the Prism framework which is heavily associated with the MVVM pattern. I've ready many different descriptions, examples and discussions on MVVM and each one is slightly different and has left me a little confused.
My understand is as follows:
The MVVM pattern has 3 parts to it :-
Model - the classes that hold the application data/information.
View - The visual elements of the application.
ViewModel - The logic, state and other behaviour associated with the visual elements. It takes data from the model and exposes it (possibly with some data conversion/formatting) in such a way that the View can use it directly.
What I'm not sure about is:
Do these 3 parts cover every part of the application? Or can there be parts of the application that are outside these 3 parts?
Is it the ViewModel or some other part that is responsible for populating the Model?
Thanks in advance
Absolutely not. Unless they do. If your application is simple, then everything can be handled in the View, ViewModel or Model(s). If your application is complex, and best practices dictate you break out logic into their own types (communication logic, repository logic, etc), then there is no stopping you. MVVM is only concerned with view-centric logic within the View, application logic within the ViewModel, and the means of storing information for transmission between the two.
The ViewModel is solely tasked with interpreting user actions and readying the results of logic within Models so that the View may display this information to the user. In some cases, it makes sense that a Model itself hold some logic so that it may respond to user actions. However, it has been my experience that this mini-ViewModel-Model design is in reaction to design decisions by inexperienced developers. Once you get the real hang of MVVM, you usually don't have to (or want to) put any code in your Models apart from validation logic.
Think of Model, ViewModel and View as logical layers that handle Business, application flow and presentation respectively. For example, a ViewModel class may delegate complex or reusable UI interactions to a separate service that doesn't correspond to any particular view but still belongs to ViewModel layer.
Yes, ViewModel stands between UI and Model.
Related
I need some help to architect an MVVM application. This time I want to do it in the right way.
My View is bound to the ViewModel which is bound to the Model. No problem there. Displaying data is not the problem, updating it is my problem.
So when the View asks the ViewModel to update the Model (with a Command), which need to handle the logic?
In my case updating the data is not trivial, there are many calculations with many files.
Also, I really want that the Model does not know the business logic. So how to do this? A property in the Model which indicates that the he wants to be updated? Or maybe the ViewModel have to know the business logic? Is it the Best practice?
The model is really a snapshot of the state of your business data. Your non-trivial calculations can be encapsulated in services that can be abstracted behind interfaces and injected into your view model. This way your view model knows nothing of these complex processes other than calling them via interface contracts. So your command fires and your view model provides orchestration, your service provides actual business calculations and an updated model would be produced representing the new state of the system.
The question is stated in the subject: are the advantages of Model-View-ViewModel (MVVM) pattern worth the overhead?
In many cases, implementing the view model involves quite significant overhead of duplicating the Model properties and sometimes synchronization between Model and ViewModel data members. For example, currently in Silverlight 4 & WCF RIA, View Models are not generated (if the developer follows the MVVM pattern, it is up to him to create the view models, often duplicating the corresponding Model's properties at ViewModel, that do nothing significant but refer to Model as the storage).
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Effectively that is what the PresentationModel is for. Which MVVM is strongly based on. The difference is that the ViewModel is the model for the view and not the model for the data. So you are concerned around more how the view behaves with the data.
If you have a simple UI that all it does is present the model then I would suggest expose the Model on a property of the ViewModel and bind to that. Make sure though the model does implement INotifyPropertyChanged etc.
The power of the ViewModel is when you have things to do in response to a user action. The ViewModel can then support Commands, calling out to services and validation and thus leaving the Model as a data container
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
In the simple cases, this is all the ViewModel is doing - wrapping up the Model so that its extended in a way that's consumable by the View. If your Model can be bound directly, you're welcome to do so.
That being said, there is more to the ViewModel layer than just wrapping the model - this is also where the application specific logic - ie: the application's plumbing, will occur. Something has to make the requests from the Model classes correctly and compose together the logic.
If you are concerned about extra work, you can always create a ViewModelBase (INotifyPropertyChanged , Errors/Validation, generic stuff) to be inherited by your ViewModel, it will minimize things that you think may cost you time to replicate. And also, Silverlight/Wpf provides us with binding which greatly reduces our coding, besides the fact that XAML also does that by providing functionalities through markup. Besides that you can further the design by using screens, controllers, etc.
For me, I do not see any "overhead" with regards to using MVVM; if there were, it'd be worth it. It properly deals with the Separation of Concerns. It provides a good platform for development especially in teams where people may take care of different aspects of the application without affecting other team members codes (especially between developers and designers).
Hope this helps
Benefits of MVVM
Reduced complexity.
Isolation of Designing and Development.
Dependency injection.
Major advantage is when you have a Well MVVM structured Windows Phone application and want to develop same for Windows Metro Desktop, Only thing u want to concentarte on design as the same view model can be used as it is.
Hope it helps.
i know some Mvvm Frameworks that introduced in this thread
please describe or give me link for that what are them useful for?
not information about MVVM about MVVM Framework.
thanks :)
i want to know :
What Is MVVM Framework?
I think your question is not really precise. As far as I understand, you ask for the features of each framework?!
You can find detailed information here and here. However, at least one of these links has already been given in the thread you mentioned...
EDIT:
Basically, an MVVM framework is a collection of classes which are commonly used in applications utilising the MVVM (Model-View-ViewModel) pattern. This may include messaging systems to communicate between independent parts of a software, dependency injection techniques, base classes for ViewModels, project/class templates, validation mechanisms, commonly used commands, techniques for displaying dialog boxes, and so on...
To completely understand such a framework, you will have to understand the MVVM pattern first. Because only then (or even only after you did your first MVVM project) you will have an understanding of the problems and/or challenges of this pattern.
To use Mvvm framework just simply follow below steps:
You have a model and a view-model with the same name.
View-models are not supposed to be wrappers around models. The job of a view-model is to broker requests for external services such as the loading and saving of data. The data itself, as well as validation and most of the business logic, should be in the models.
I can’t emphasize this enough. Whenever you create a view-model that wraps a model by delegation you introduce a huge hole in your API. Specially, anything with a direct reference to the model can change a property in such a way that the view-model and thus the UI are never notified. Likewise, any changes to calculated fields in the model won’t be propagated back to the view-model.
You have a view and a view-model with the same name.
Ideally view-models are agnostic to the screens they are used by. This is especially true in a WPF application where multiple windows may be sharing the same instance of a view-model.
For smaller applications such you may only need a single view-model for the whole application. For larger applications you may need one for the main functionality and one for each secondary aspect such as configuration management.
You have no code behind.
In absolute terms code behind is neither a good nor a bad thing. It is merely a place to put logic that is specific to a single view or control. So when I see a view with no code-behind at all I immediately check for the following mistakes:
Does the view-model touch specific controls by name?
Is the view-model being given access to controls via a command parameter?
Is EventToCommand or another leaky behavior being used in place of simple event handler?
EventToCommand from MVVM Light is especially bad because it will prevent controls from being garbage collected after they are removed from the screen.
View-models are listening to property changed notifications
If a model has a longer life-span then the view-model that listens to its events then you probably have a memory leak. Unlike views which have an unloaded event, view-models don’t have a good story for life-cycle management. So if they attach an event to a model that may out-last them then the view-model will be leaked.
When I'm designing multiple views under the MVVM pattern, does each view get its own ViewModel or do they all share the same one? I understand that this is ultimately a flexible decision, but what is the best practice?
My gut tells me to have a ViewModel for each view (i.e. each separate UI window). All of the blog examples of MVVM show a single view but not much beyond that.
Yes, basically the idea is that your viewModel should only be used by one view. If you use a viewModel to populate an area (like in ASP.NET MVC) then that viewModel is "reused" each time that view is presented in difference places.
This article is a discussion by Josh Smith of the MVVM pattern. Then, Ward Bell discusses in this article what he thinks Josh left out, while maintaining that Josh's work is still very strong.
Ward does an excellent job of laying out the complexities of this pattern and showing the tension that exists. Here is his take on the tension:
In my experience there is a “dialog” between View and ViewModel design. The VM exists to serve a view even as it strives for independence from any particular concrete view. A VM is useless if there is no view that will work with it; clearly the VM developer must
heed the imprecations of the View developer.
On the other hand, in business applications the application’s imperatives – what the view must do to satisfy business requirements – are the province of the programmer and are > best articulated through the capabilities of the ViewModel.
Therein lies the necessary tension between View and ViewModel design. As a developer my allegiance is with the ViewModel (“the application should do something worthwhile”) but it would be silly to defend that allegiance at the expense of the View (“a good UX is essential to making an application easy to learn and to use”).
To me thats a subjective statement- text book, I would say definately a 1-1 pairing - and there is certainly nothing wrong w/ being proactive and setting the paradigm by having a 1-1. However, if you have multiple views each representing a slice of the same data you dont necessarily have to have 1 per- you could reuse the same one across multiple views until you had a deviation. The purpose of the view model being a bridge between the ui and the business layer...if the business functions are the same you are either going to have a common view model interface or base and cast down or you are going to be replicating the same logic multiple times. If the view is the only thing that changes I see no issue in reusing the same view model until there is deviation.
In wpf, you should be binding to values on the model so unless your view model has a reference to your concrete view that shouldnt be an issue. Even it does, you could abstract out to a view contract (interface) and change the property to be that type instead.
Yup, each view should have its on ViewModel.
I don't know WPF inside out, but generally the ViewModel mediates between a UI component and a business logic component. In other words: It's specific to a view/model pair - this is the only reason for this component to exist...
HTH.
This question already has answers here:
Why use MVVM? [closed]
(13 answers)
Closed 2 years ago.
Why we go for MVVM over MVC or MVP while dealing with WPF?
What extra benefit we get by using this?
Edit:
To be honest , today I had an interview and I have been asked this question. I answered like INotifyPropertyChanged , ICommand,IValue Convertor.. but he was not satisfied. Henceforth I have put up this question
Thanks in advance
I'll point you to a particularly useful video by Jason Dolinger.
Coming from a WinForms world, implementing any MVX style pattern seemed like more hassle than it was worth but after working with WPF for a couple of years now, I can honestly say that I wouldn't consider anything less. The whole paradigm is supported out-of-the-box.
First off, the key benefit is enabling true separation between the view and model. What that means in real terms is that if/when your model needs to change, it can without the view needing to and vice-versa.
Secondly, while your model may contain all the data you might need in your view, you may want to abstract that data in such a way that your model doesn't support. For example, say your model contains a date property. In the model it can exist solely as a DateTime object but your view might want to present it in a completely different way. Without the viewmodel you'd either have to duplicate the property in the model to support the view or modify the property which could seriously obfuscate the 'model'.
You can also use a viewmodel to aggregate parts of your model that exist in separate classes/libraries to facilitate a more fluent interface for the view to deal with. It's very unlikely that you'll want to work with data in your code in the same way that a user will want to or will want that data presented to them.
On top of that, you get support for automatic two-way data binding between the view and viewmodel.
There really is a whole bunch of extra stuff that I could bang on about but Jason say's it far better that I could so my advice is watch the video. After a few days of working like this, you'll wonder how you ever got by without it.
Good luck.
These are mine specific to MVVM
Increases the "Blendability" of your views (ability to use Expression Blend to design views). This enables a separation of responsibilities on teams that are lucky enough to have a designer and a programmer... each can work independent of the other.
"Lookless" view logic. Views are agnostic from the code that runs behind them, enabling the same view logic to be reused across multiple views or have a view easily retooled or replaced. Seperates concerns between "behavior" and "style".
No duplicated code to update views. In code-behind you will see a lot of calls to "myLabel.Text = newValue" sprinkled everywhere. With MVVM you can be assured the view is updated appropriately just by setting the underlying property and all view side-effects thereof.
Testability. Since your logic is completely agnostic of your view (no "myLabel.Text" references), unit testing is made easy. You can test the behavior of a ViewModel without involving its view. This also enabled test-driven development of view behavior, which is almost impossible using code-behind.
The other two patterns are really sort of separate in terms of the concerns they address. You can use MVVM with MVP and MVC (most good samples out there do some form of this).
In fact, MVP (w/ a Passive View, rather than a Supervising Controller) is really just a variant of MVVM, in my opinion.
WPF has better databinding than any other UI framework, which MVVM would be unruly without
MVVM provides unit testability and excellent view-agnosticism, which make it a good thing to use
Baked in support for ICommand and INotifyPropertyChanged are the two biggest benefits. Using MVVM makes it really easy to wire up the commands and plug data into the WPF UI. Things just work.
I personnaly see MVVM not as a benefit, but as an obligation for those who want to use WPF cool features.
WPF is very very heavily built with data binding at the core, to enable separation of UI from Model. But the way data binding is technically done in WPF is somewhat special, as it's tied to classes like:
DependencyProperty
INotifyPropertyChanged
ObservableCollection
Because of this you just can't really write a model the way you want using standard .NET technology. For example, the WPF TreeView is almost impossible to use w/o using data binding and templates. You just can't populate it simply like you would from a generic model in Winforms for example. It must be bound to a hierarchical model using ObservableCollection to represent a node's children.
So let's say V represents the XAML code and it's code-behind counterpart (so it's tied to WPF as a technology), and let's say M represents your model (so it's not tied to WPF UI technology in anyway).
Well, you'll never have this working properly under WPF with only these V & M.
You must add something between the two. Something that's WPF-compatible and understands your model. Something that speaks DependencyProperty, ObservableCollection and INotifyPropertyChanged. That's what's called VM.
As a side note, an alternative to MVVM is to build a V & M (w/o VM plumbing) combination with M being WPF-compatible but still with a reasonable UI independency. Historically, ObservableCollection was in the WindowsBase.dll assembly (that was shipped with WPF), so it really looked weird to bind a generic model to something tied to a UI technology. It's been moved back to System.dll since. Even then, it's sometimes hard to keep a pure VM model w/o tweaking the M specifically for WPF...
The ability of XAML code to databind, as well as the existance of triggers will break the MVP and MVC Patterns.